Protein C is a vitamin K dependent plasma protein. Protein Ca, the enzymatically active form, is a potent anticoagulant functioning at the levels of factors V and VIII in the clotting cascade. Protein Ca also accelerates the rate of blood clot lysis at least in part by elevating the levels circulating plasminogen activator. Because of these unique properties we wish to determine the importance of protein C in human thrombotic disease. To accomplish this long-term goal we propose to develop both screening assays for the protein and quantitative functional assays for the protein. We will develop a functional assay for plasma levels of protein Ca, and for another vitamin K dependent plasma protein, protein S, which appears essential for expression of the anticoagulant properties of protein Ca. Using these assays we will investigate conditions in which the levels of these proteins are altered: 1) We have previously demonstrated that low-level thrombin infusion in dogs results in the activation of Protein C. We will determine if activation of Protein C occurs in humans following the tissue trauma of general surgery, crush injury or during the intravasclar clotting associated with disseminated intravascular coagulation. In the surgery patients we will determine if protein C activation protects the patients from postoperative venous thrombosis. 2) The commonly used oral anticoagulant warfarin lowers protein C levels and we will determine the relationship of the recovery of protein C and protein S to that of factors VII, IX and X when warfarin therapy is discontinued. 3) Using a canine model we will determine if protein C supplementation will protect animals from disseminated intravascular coagulation induced by the continuous infusion of tissue thromboplastin. These human and animal studies will be an index of the importance of induced changes in protein C and protein S levels in human thrombotic disease.